Agarwood is a highly prized incense that is extremely rare. It has at least a 3000-year history in the Middle East, China and Japan. There are also references to agarwood in the literature of India and France, and even in the Old Testament of the Bible. Agarwood remains today the world's most expensive incense. The value of agarwood shipped out of Singapore alone each year has been estimated to exceed $1.2 billion. E. Hansen, Saudi Aramco World 51:2-13 (December 2000). This aromatic resinous wood has many common names including agarwood, gaharu, eaglewood, aloeswood, agila wood, aguru, agar, oud, ude, ud, ood, oode, jinkoh, jinko, Ch'Ing Kuei Hsiang, Ch'En Hsiang, Chan Hsiang, Chi Ku Hsiang, Huang Shu Hsiang, kalambak, and grindsanah.
The resin is used by Traditional Chinese, Unanai, Ayuravedic, and Tibetan physicians. Used medicinally, agarwood is a remedy for nervous disorders such as neurosis, obsessive behavior, and exhaustion. Agarwood is highly psychoactive and is used in spiritual rituals. Many religious groups prize it as a meditation incense, to calm the mind and spirit. In Ayuravedic medicine it is used to treat a wide range of mental illness and to drive evil spirits away. In Japan, it is considered by many to be sacred, and is used to anoint the dead. In Buddhism, it serves as a major ingredient in many incense mixtures, and it is considered to be one of the three integral incenses, together with sandalwood and cloves.
The source of agarwood is the Aquilaria tree. The Aquilaria tree is an evergreen that grows up to 40 meters high and 60 centimeters in diameter. It bears white flowers that are sweetly scented. The genus Aquilaria is an angiosperm taxonomically placed in the Thymelaceaceae family. Fifteen species of Aquilaria have been reported and all produce agarwood. The taxonomy of these species is not completely clear and not all species are recognized by taxonomists. Species include Aquilaria malaccensis, A. agallocha, A. baillonii, A. crassna, A. hirta, A. rostrata, A. beccariana, A. cummingiana, A. filaria, A. khasiana, A. microcarpa, A. grandiflora, A. chinesis or A. sinensis, A. borneensis, and A. bancana. Aquilaria bancana has been questioned as a true species of Aquilaria and has been placed in the Gonystylaceae family of the Gonystylus genus, as Gonystylus bancanus. Gonystylus has also been found to produce an aromatic resin that is considered the same as or very similar to agarwood.
Aquilaria trees are native to Asia from Northern India to Vietnam and Indonesia. The healthy wood of the Aquilaria tree is white, soft, even-grained, and not scented when freshly cut. Under certain pathological conditions, the heartwood becomes saturated with resin, and eventually becomes hard to very hard. The best grade of agarwood is hard, nearly black and sinks when placed in water. In general, agarwood is considered inferior as it appears lighter in tone, with diminishing amounts of resin.
The process of agar deposits is not fully understood. A Dutch paper from 1933 (J. P. Schuitemaker, “Het garoehout van West Boreno” Boschbouwkundig Tijdschrift Tectona Uitgave der Vereeniging van Hoogere Ambtenaren bij het Boschwezen in Nederlands Oost-Indi 26:851-892) reported the occurrence of agarwood in Borneo and discussed many different types of resin produced in trees. Most of the local people at the time believed that agarwood formed from mysterious ways and was associated with the spirit world. The author stated that “the mysterious occurrence of the ‘holy’ wood is connected to supernatural powers” and that agarwood was referred to as “wood of the gods.” The author also noted that “we cannot exclude the possibility of a pathological occurrence of which the cause was unknown,” that “perfect trees never have agarwood,” and that agarwood “is formed around wounded or rotting parts of the trunk.” The author also suggested that salt put into holes in trees might promote resin. The paper also stated that if the agarwood was infectious, maybe it would be possible to induce agarwood formation by infecting the trunk artificially by putting fresh cut agarwood into the stem.
Later authors also reported the longstanding belief was that agar deposits were created as an immune response by the tree, the result of attack by a fungus. I. H. Burkill, A dictionary of the economic products of the Malay Peninsula. Vol. I Crown Agents for the Colonies, London p. 197-205. In the 1940's and 1950's, several researchers investigated the origins of agar deposits with varying and sometimes conflicting results. Rahman and Basak, Bano Biggyan Patrika 9:87-93 (1980). Others concluded that it was unlikely that there was a specific fungal cause for the production of agarwood. Gibson, Bano Biggyan Patrika 6:16-26 (1977). It was suggested that the resin deposits might arise as a direct response of the stem tissues of the tree to wounds with subsequent invasion by weak pathogens. Id.
Rahman and Basak suggested that wounding produced color changes in the wood with some “oleoresin” deposits. Rahman and Basak, Bano Biggyan Patrika 9:87-93 (1980). They postulated that the presence of an exposed, open wound seemed to be of more importance than the presence of certain species of fungi within a wound. They, however, concluded their paper by stating that further investigation was needed in order to determine what factors were responsible in wounding, which are important in agar deposition.
The identification of the small proportion of the trees having agar is difficult and destructive, which added greatly to the near-extinction of natural stands of tress. Also, large-scale logging operations have destroyed many forested areas where the Aquilaria trees are found. Thus, the current source of agarwood, the naturally-growing old-growth Aquilaria trees, is becoming extinct. To date no one has successfully cultivated agarwood. E. Hansen, Saudi Aramco World 51:2-13 (December, 2000). Therefore, there is a growing need for a means to cultivate Aquilaria trees that produce agarwood as a renewable source for agarwood.